Intermolecular CH⋯O hydrogen bonds in formyl-substituted diphenylhexatriene, a [2+2] photoreactive organic solid: Crystal structure and IR, NMR spectroscopic evidence

Abstract

(E,E,E)-1,6-di(4-formylphenyl)-1,3,5-hexatriene (1) undergoes [2 + 2] photocycloaddition in the solid state, while the unsubstituted parent, (E,E,E)-1,6-diphenyl-1,3,5-hexatriene (DPH), is photochemically inert as we reported previously. Although the steering effect of the formyl group for the photoreaction is very clear, the crystal structure of 1 had been unknown. In this study, single crystals of 1 suitable for X-ray diffraction (XRD) analysis were prepared and the crystal structure was investigated. The molecules in crystal are linked through intermolecular CH⋯O-type hydrogen bonds involving formyl groups to form a 3D structure of π-stacked crystal packing arrangement with intermolecular distance of 3.926 Å (=b), less than the Schmidt’s criteria of 4.2 Å for the [2 + 2] photoreaction. The existence of CH⋯O hydrogen bonds is evidenced by FT-IR, 1H and 13C NMR spectroscopic measurements. For the IR peak assignments, ab initio and DFT calculations were performed. The solid-state 1H and 13C NMR spectra were measured using the CRAMPS and CP/MAS techniques, respectively. Owing to the small CH⋯O angular dependence of the interaction energy (directionality), the CH⋯O hydrogen bonds are formed in a multiple manner. Thus, although being rather weak, these hydrogen bonds play a decisive role in constructing the [2 + 2] photoreactive crystal structure, without any other stronger intermolecular interactions coexisting.